Estufa means "stove" in Mexican Spanish, but the canyon was not living up to
its name when we began our hike. Cold weather is not unusual in Big Bend during
December (we were at Estufa Canyon on the 28th), but low temperatures were in
the twenties that day. A complication to the below-average temperatures was the
prospect of a winter storm bearing down on West Texas, with several inches of
snow forecast. My brother, Randy, and I headed down the trail wondering if we'd
need to improvise snow shoes to make it back. In spite of the ominous forecast,
the clouds stayed away on our first day in the canyon, allowing the air to warm
up for a pleasant afternoon's hike. (I did this hike again in April, 2014, this
time alone. My brother, Randy, was off in other parts of the park birding.
Some of the material that follows was from that hike.)

The following picture was taken on the
first trip, looking west. To the left and right of that fine specimen of
manhood in the photo is a line of
intrusive
igneous rock, possibly a
sill, possibly
related to the rings of igneous
rock associated with the Pine Canyon
Caldera, whose remains are
on the other side of Pummel Peak. If it is a sill (a sheet-like
intrusion of igneous rock that follows, in this case, pre-existing bedding
planes), then it is resting on top of the Black Peaks formation. At any rate,
the Black Peaks formation, is at the base of the ridge, as shown in the picture.
The Black Peaks consists mostly of non-marine mudstone and sandstone, and was
deposited from the late
Cretaceous into the
Paleocene. The surface on
which our hero stands
consists of desert
pavement lying on top of
Pleistocenealluvium (water-deposited
sediment). Behind Sir Totes-a-Lot,
out of view between the igneous outcrops, is the Chihuahuan Desert
Research Station. On my second trip to Estufa Canyon, there were researchers
there tracking radio-collared mountain lions. They could tell when one made a
kill. Then they would go to that location to see what the lion had eaten.
(Fortunately, only occasionally a backpacker ;-) ). Pretty interesting
stuff.

If you turn around and look to the east, you see the sediments out of which
Estufa Canyon was carved and, in the far distance, the Sierra del Carmen.
Estufa Canyon is in the Estufa bolson just to the east of the Chisos
mountains. A bolson is a large, flat-floored basin often formed by a block of
the Earth's crust that has subsided between surrounding faults. It is a
topographical term generally restricted to the
Basin and Range
Province of North America. The down-faulted block responsible for this
bolson is called the
Tornillo graben. Several
drainages from off the
pediment on the
east side of the Chisos Mountains combine to form the drainage that has
eroded down through the sediment, creating Estufa Canyon.

In the image below please ignore my brother and
direct your attention to the topography behind him. (The view is toward the
northeast.) The low area running from
left to right on this side of the hills in the background is where these
drainages come together. One drainage descends from the left (NW to SE) along
this side of the hills, combining with others draining from the west. Before
they join to cut through the hills, carving out Estufa as they head for
Tornillo Creek, their erosive forces have exposed the
Eocene Canoe formation
beneath the geologically recent alluvium of
Quaternary age,
designated Qya (after Berry et al. in
USGS Circular 1327).
The hills in the background are underlain by the (informal) Estufa Canyon
formation (designated QTa, again after Berry et al.). These are older
alluvial deposits, dating back to as old as the
Pliocene, although their
actual age is not well established. These are the sediments that form the
walls of the canyon.

The Canoe formation marks the beginning of volcanic activity in Big Bend,
as it consists of deposits of
tuff (rock composed of volcanic
ash) along with clay, mud, sand, and a few
basalt flows. The mud and
clay are "tuffaceous" (containing volcanic ash). In the image above, the Canoe
lies beneath the Quaternary alluvium with its white and reddish beds
at an angle to the bed above, forming an angular
unconformity. However,
check out the following picture I took on my second trip down the canyon. It
appears that the beds of the Canoe trend toward horizontal to the left.
As you will
see farther down the trail, the Canoe and alluvial beds seem to become
parallel. (Without doing careful mapping, however, you can be fooled by your
perspective.) After considering what I saw in my second trip down, I think that
the Canoe may have an undulating bedding pattern in this area: generally
horizontal but with slight ups and downs. Additionally, the beds may have been
affected by geologically recent
Basin and Range
faulting that created the Tornillo graben. The
Miocene rocks that are
exposed on the east end of Estufa Canyon below the alluvial beds are absent
here. Were they present, they would lie above the Canoe and below the more
recent alluvium. They must have once been here, as indicated by their
presence at the west end of the graben at other locations, but were victims
of erosion.

The dashed line in the photo above, graced by the visage of my bro, marks a
fault belonging to
the north-south trending Dugout Wells fault zone that establishes the western
boundary of Tornillo graben, where the side we are on has moved up with respect
to the far side. This is shown by the
presence of the much older Canoe formation at about the same elevation as the
younger alluvial deposits that underlie the hills. If erosion eats down
fairly evenly on both sides of a steep
dip-slip fault,
older rocks on the upside of the fault will be seen at the same topographic
elevation as younger rocks on the downside. The latest movement on
this (Basin and Range) fault has to be relatively young, as it cuts through the
Estufa Canyon sediments and is therefore younger than these. The western
boundary of Tornillo graben is kind of a
mess, but consists approximately of a series of N-S trending
normal
faults on the west side of the Sierra del Carmen.

The following image is of an outcrop of the Canoe formation composed of
volcanic, gray-colored tuff. It is not well-consolidated and is probably
heavily weathered but does
contain relatively large crystals (not visible in this picture) in a
fine-crystalline background called
phenocrysts. These appear
to be
feldspar due to their light
color and rectangular shape.

Here is a photo taken farther down the drainage that spills into Estufa
Canyon. This seems to be the same type of rock shown in the previous photo. The
rock is quite degraded; however, the phenocrysts of feldspar are still present
and visible. The pocket knife is there for scale.

As you hike down the drainage toward Estufa Canyon, you descend
stratigraphically
through the Canoe formation until you cross the fault
mentioned above. In the following image you see the younger alluvium atop the
older Canoe formation. Here, the Canoe appears to contain a lot of tuff and/or
tuff-bearing sediment. Also, as pointed out above, the Canoe beds now appear to
be more or less parallel to the alluvium. The parallel pattern was also seen in
other locations farther down the same drainage.

After you cross the fault, you are in the informal Estufa
Canyon formation of Stevens and Stevens (Bulletin of the American Museum of
Natural History, vol 279, p 177). ("Informal" means that it hasn't been
generally recognized by the stratigraphic community, but often informal units
are formalized if the work done in defining them stands the test of time and
conforms with accepted code.) This
sediment is thought to be basin fill derived from the direction of today's
Chisos Mountains, deposited by flash floods on
alluvial fans from the
late Pliocene through the early Pleistocene (Berry et al).

The picture below shows an outcrop of this formation in Estufa Canyon, just
to the left of a cave (not seen in this photo) apparently formed by
erosion of flash-flood waters. The rounded gravel and cobbles indicate they
were weathered over time by running water, probably during a wetter climate
in Big Bend. However, the sediment is very poorly sorted, indicating it
was deposited rather rapidly in its current resting place. The rounded
clasts ("clast" is
essentially the geology term for originally loose
fragments of rock) and the poor sorting likely indicate an extended period of
time when the clasts were weathered, followed by a violent deposition during a
flash flood on an alluvial fan, and then perhaps further weathering until being
buried by subsequent flash flood debris.

A sheet of
calcite is located above my
brother Randy in the photo above. Accumulation of calcite is usually part of
the evolution of
soil in relatively dry climates. Geologists classify this evolution in four
stages, where stage III and IV are the oldest. The sediments we encountered were
well-cemented with calcite (stage III) as shown here, but the sheet of calcite
indicates this sediment has reached stage IV. The advanced calcite evolution is
the reason this sediment is thought to be several hundred thousand years
old.

The calcite sheet mentioned above is in sort of an "island bar" in the
canyon, in this case an
erosional remnant where the currents have eaten down deeper on either side. A
cave has been formed by water undercutting the "island". Note that the calcite
sheet appears to have formed by deposition in a fracture in the rock. The sheet
has probably been exposed by failure along the fracture, such that the rock
on the camera side of the fracture has fallen away. You can also tell in this
picture that the beds are
dipping to the east
(since the photo is taken facing
the Sun in the south). Recall the beds on the west side of the Dugout Wells
fault zone were virtually horizontal. Farther down the canyon the beds will
become close to horizontal once again. The dip of these rocks is probably due
to their being just to the east of the Dugout Wells fault.

Below you see some of the nice calcite crystals found in the rocks of the
Estufa formation. Calcite seems to be almost everywhere in Big Bend. (If you
happen to come across crystals like these, please leave them for others to
enjoy. Also it is unlawful to collect in Big Bend.)

The informal Estufa Canyon formation is a
conglomerate.
As indicated above, it appears to be due to many hundreds of thousands of years
of sand, gravel, and cobbles being washed into a low lying basin, the Estufa
bolson. Sand is moved fairly easily by swiftly running water, but gravel and
cobbles are difficult to move and only move appreciably during periods of very
strong flow such as flash floods. Gradually, these are buried beneath more sand,
gravel, and cobbles transported by later episodes of moving water. Now that the
area is subject to net erosion rather than net deposition, the work of
countless years of flood waters is exposed in Estufa Canyon.

The picture below shows a cliff of the Estufa Canyon formation. Note the
bedded pattern, irregular but roughly parallel, which implies water deposition.
In the inset you can see a detail of the depositional pattern. The line marked
"1" indicates cross bedding, which could be due to the removal of earlier
material by water action, followed by new deposition. Also note ("2") that finer
material alternates with coarser material, indicating changes in the speed of
the sediment-carrying water. For example, a fast flow may erode much of the
finer clasts, leaving the coarser clasts behind, or, in another instance, may
be strong enough to transport sand and clay, but not gravel. A slowing of water
speed may allow finer material to be deposited. Hence, the coarseness of the
clasts in sediment is an indicator of the speed of the water that deposited
them.

Next is a photo of a lens of fine sand and clay in either the Estufa
formation or possibly the unit designated as "Qia", late to middle Pleistocene
alluvial sediments, in the new USGS map (Scientific Investigations Map
3142, 2011). (These sediments look a lot alike to someone who is not an
experienced sedimentary petrologist and both are found in the canyon.) Perhaps
this was originally a swale (a marshy depression) that was eventually covered
by debris from a flash flood. As you can see, some of the clasts are boulder
size.

Farther down the canyon, the canyon walls get higher and the erosional
topography more severe (at least as viewed from the canyon floor). The
following photo is reminiscent of
badland topography, and the
satellite view of this area shows
dendritic topography, where the drainage pattern
looks like the growth pattern of plant roots.

The walls of the canyon can reach heights close to 100 feet. Not as
impressive as Santa Elena Canyon, but the erosional beauty makes up for that.
The following is a typical view about a mile and a half from Tornillo Creek.

As you approach Tornillo Creek, you descend stratigraphically into older
rocks. Below you see what I take as the Estufa Canyon formation overlying the
Banta Shut-in formation of Stevens and Stevens of late
Miocene age. (The Miocene
occurred between 23 and 5 million years ago.) The Banta Shut-in formation is
noted for its Miocene mammal fossils
(Stevens and Stevens). Although the overlying sediment
looks very much like the Estufa Canyon formation farther west, note that it
consists of fine sediment compared to
the conglomeritic texture seen in previous images above. It could be that all
this sediment belongs to the Banta Shut-in formation. However, I'm going to
stick with the Estufa over Banta Shut-in interpretation for now. I would like
to get back here in the future and spend more time in the canyon itself.

Although the contact here could be interpreted as indicating a
sudden change in depositional environment, other areas (Stevens and Stevens)
have shown the contact to be an erosional one with apparently a significant
time gap between the deposition of the Miocene Banta Shut-in and the Pliocene
to Pleistocene Estufa Canyon formations. The next image appears to bear this
out (assuming my identification of the formations is correct), as an uneven
surface between the two formations is clearly evident. Note how the clasts get
coarser higher in the Estufa Canyon (?) section. This could be due to increasing
topographical relief, an
increase in precipitation, or both. A break in deposition is called
an unconformity. Since the sedimentary beds appear to be parallel, or nearly so,
above and below the contact, this would probably be designated a type of
unconformity called a
disconformity,
as indicated in the two pictures below.

As you approach Tornillo Creek, the erosion becomes more advanced. Below you
see an erosionally formed column containing what I suppose is the Estufa
Canyon-Banta Shut-in contact. The Banta Shut-in formation was
divided up into several "members" by Stevens and Stevens. The rock seen in the
picture below and those above belong to the Bench member. If my supposition is
incorrect, the contact could be between the Bench member and the Lower member
(see below).

Now here's a weird sight. Erosion has eaten through a thin wall in
the Banta Shut-in formation, creating what appear to be cannon holes, and yet
another column of rock, like the one pictured above, is apparently in the
making. Since no battles were ever fought in this area to my knowledge, I guess
these are really "canyon holes" (yuk, yuk). The picture follows my tentative
interpretation of the identity of the beds.

It should be obvious to the reader that I am a bit confused over the
stratigraphy in this area. According to Stevens and Stevens, as you leave the
canyon and approach Tornillo Creek (as we were doing when the above image was
taken), you cross first the Bench member of the Banta Shut-in formation, then
the Lower member of this formation. These units are described in their paper
by stratigraphic sections but not much verbal description, and none of their
stratigraphic sections were drawn at this location. The Bench member is said to
be 60 meters thick (about 200 feet). From the elevations of the Roys Peak
quadrangle, the top of the Banta Shut-in
in the above photo is about 200 feet above Tornillo Creek. So, are the following
pictures, taken about 4000 feet west of the creek at an elevation much less
than 200 feet below the top of the Bench member, of the contact between the
Bench and Lower member? If not, I couldn't really find anything I thought might
be the Lower member between the location above and the creek, although
certainly the Lower member could be mostly covered with debris and difficult to
find. However, you could also contend that the conglomerate-clay contact
shown in previous pictures is really the contact between the Bench member and
the Lower member, even though the conglomerate sure looks like Estufa. If that
is the case, then the varicolored rock in the images below might be the Aguja
Formation of upper Cretaceous age or even an isolated outcrop of the upper
Cretaceous Javelina Formation, although it is not mapped in this area. The
problem with the conglomerate being the Bench member is that
the Bench member is depicted as mostly non-conglomeritic in the stratigraphic
sections of Stevens and Stevens. I would have to spend more time exploring the
canyon to convince myself one way or the other.

Finally, turning from geology to botany, here are a couple of pictures for
you flower lovers. Both were taken on my second trip down Esufa during April.
The first is a blooming ocotillo, just before you descend into the drainage that
runs into Estufa Canyon. The second was taken later, of a flowering mesquite
bush. The insects were having a field day collecting nectar.